Photochemical Production and Biological Consumption of Carbon Monoxide (CO) in the Sea Surface Microlayer of Temperate Coastal Waters: Implications for Air-sea CO Exchange
In the SML, the light-normalized photochemical CO production rate was relatively high from spring to autumn (3.85 ± 3.09 nM [kWh m–2]–1) when relatively high absorbance of chromophoric dissolved organic matter (CDOM) (0.69 ± 0.38 m–1) was observed. Biological CO consumption rate constant in the SML showed relatively high values from spring to autumn (0.060 ± 0.010 h–1) during the period of relatively high water temperature (22.3 ± 2.7°C). Although the calculated sea-air CO flux (F) varied similarly to CO concentration ([CO]) in the subsurface water, [CO] was particularly high in the SML (15.0 nM) during a phytoplankton bloom there, which indicates the suppression of air-sea gas exchange by the accumulation of biogenic surfactants in the SML.
The turnover time of the photochemical production (τprod), biological consumption (τcons), and sea-air emission (τsea-air) of CO in the SML were compared. τcons (19.0 ± 5.0 h) was about three orders of magnitude higher compared to τsea-air (0.024 ± 0.024 h) during the study period, suggesting that biological consumption in the SML can be ignored in air-sea CO exchange throughout the year. However, although τsea-air/τprod ratio was low most of the year (< 3.03%), τprod was relatively comparable to τsea-air in August 2017 with τsea-air/τprod ratio of 21.9%, which suggests that photochemical production in the SML enhances F during summer.